Glutamate is the primary excitatory neurotransmitter in mammalian brain. Glutamatergic signaling participates in a wide range of neural functions including learning and memory, long-term potentiation and synaptic plasticity.
Glutamate receptors can be divided into two families. The ionotropic glutamate receptors form ion channels that activate upon binding agonist, opening a pore through the plasma membrane through which cations can flow. The metabotropic glutamate receptors are G-protein-coupled receptors, activating intracellular signal transduction cascades. The ionotropic glutamate receptors can be further subdivided into four sub-families, based upon sequence homology and selectivity to exogenous agonists. These sub-families are the AMPA (α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionic acid), NMDA (N-methyl-D-aspartate), kainate, and delta receptors.
The AMPA subtype of glutamate receptors are glutamate-gated ion channels expressed primarily on postsynaptic membranes of excitatory synapses in the central nervous system. AMPA receptors assemble as tetramers of subunits. Mammals express four AMPA-receptor subunits, called GluA1-GluA4. Each GluA subunit can be expressed in multiple splice variants; the two most prominent splice variants are called flop and flip. GluA subunits freely form functional homo- and hetero-tetramers. The majority of RNA encoding GluA2 subunits is edited post-transcriptionally, altering a genetically-encoded glutamine to arginine. This RNA editing causes AMPA receptors to preferentially form with two GluA2 units, and also prevents calcium entry through the activated receptor.
In their native environment, the pore-forming GluA tetramers directly or indirectly associate with numerous auxiliary proteins which modify the trafficking, localization, gating characteristics, and pharmacology of the AMPA receptor (AMPAR). These auxiliary subunits include cytoskeletal and anchoring proteins, other signaling proteins, and several intracellular and transmembrane proteins with unknown function. The wide variety of proteins which can participate in AMPA receptor complexes vastly increases the ability of a neuron to tune the response characteristics of its synapses.
Transmembrane AMPA Receptor Regulatory Proteins (TARPs) are a fairly recently discovered family of proteins that have been found to associate with and modulate the activity of AMPA receptors. (Gill and Bredt., Neuropsychopharmacology 36(1): 362-363 (2011). Several TARPs exhibit regiospecific expression in the brain, leading to physiological differentiation of the AMPA receptor activity. For example, TARP γ2-dependent AMPA receptors are primarily localized in the cerebellum and cerebral cortex while TARP γ8-dependent AMPA receptors are localized primarily in the hippocampus.
AMPA receptors mediate the majority of fast neurotransmission across synaptic gaps. Thus, inhibition or negative modulation of AMPA receptors is an attractive strategy for therapeutic intervention in CNS disorders characterized by excessive neuronal activity. However, since AMPA receptor activity is so ubiquitous within CNS, general antagonism affects most areas of the CNS resulting in undesired effects, such as ataxia, sedation, and/or dizziness, which are shared by all known general AMPA receptor antagonists.
Epilepsy affects over 50 million people world-wide, with 30-40% of treated patients being resistant to current pharmacotherapies and only about 8% of treated patients being maintained seizure free. Epilepsy is often defined as when a person has two or more unprovoked epileptic seizures. The International League Against Epilepsy (ILAE) defines an epileptic seizure as “a transient occurrence of signs and/or symptoms due to abnormal excessive or synchronous neuronal activity in the brain.” Seizures are thought to have a number of underlying causalities which adds to the difficulty in treating epilepsy. Seizures have been divided according to their clinical presentation including generalized seizures (absence, atonic, tonic-clonic (grand mal), and myoclonic), simple and complex partial onset seizures, gelastic seizures, dacrystic seizures, and status epilepticus. Current therapies target a variety of mechanisms including GABA γ-aminobutyric acid) receptor agonism, T-type calcium channel blockers, sodium channel modulators, synaptic vesicle protein SV2A modulation, and inhibition of GABA transaminase. More recently, AMPA receptor antagonists have been investigated for treatment of seizures as well.
AMPA receptor antagonists are known anticonvulsant agents. Typically, AMPA receptor antagonists have very narrow therapeutic dosing windows; the doses needed to obtain anti-convulsant activity are close to or overlap with doses at which undesired effects are observed. (Michael A. Rogawski. “Revisiting AMPA Receptors as an AntiEpileptic Drug Target” Epilepsy Currents 11.2 (2011).) However, certain anticonvulsant agents such as Talampanel ((8R)-7-Acetyl-5-(4-aminophenyl)-8,9-dihydro-8-methyl-7H-1,3-dioxolo[4,5-h][2,3]benzodiazepine), selurampanel (BGG492) (N-[7-isopropyl-6-(2-methyl-2H-pyrazol-3-yl)-2,4-dioxo-1,4-dihydro-2H-qui-nazolin-3-yl]methanesulfonamide), and perampanel (5′-(2-cyanophenyl)-1′-phenyl-2,3′-bipyridinyl-6′(1′H)-one) are general (non-TARP dependent/non-selective) AMPA receptor antagonists. However, such general antagonism affects most areas of the CNS resulting in undesired effects.
Glutamate as an excitatory neurotransmitter has been known to induce neurotoxicity by, for example, abnormal excitation of central nerves. Neurotoxicity is an adverse structural or functional change in the nervous system, and can take the form of subtle or gross biochemical changes, axonal degeneration, dendritic pruning or sprouting, loss or rearrangement of synapses, or cell death. Numerous nervous diseases involve a neurotoxic component, including and not limited to cerebral ischemia, head injury, spinal cord injury, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis (ALS), Huntington's chorea, AIDS nervous disturbance, epilepsy, mental disorder, mobility disturbance, pain, spasticity, nervous disturbance by toxin in food, various neurodegenerative diseases, various mental diseases, chronic pain, migraine, cancer pain and diabetic neuropathy.
Substances showing an antagonistic action to excitatory neurotransmitter receptors are potentially useful for the treatment of the above-mentioned conditions. For example, WO2000001376 suggests that inhibitors of the interaction of glutamate with the AMPA and/or kainate receptor complex could be useful in treating demyelinating disorders such as encephalitis, acute disseminated encephalomyelitis, acute demyelinating polyneuropathy (Guillain Barre syndrome), chronic inflammatory demyelinating polyneuropathy, multiple sclerosis, Marchifava-Bignami disease, central pontine myelinolysis, Devic syndrome, Balo disease, HIV- or HTLV-myelopathy, progressive multifocal leucoencephalopathy, a secondary demyelinating disorder; for example, CNS lupus erythematodes, polyarteritis nodosa, Sjogren syndrome, sarcoidosis, isolated cerebral vasculitis, etc.
Hippocampus links the limbic system to frontal cortex, thereby linking emotion to cognition (Small et al, Nat. Rev. Neurosci. 12:585-601, 2011). A meta-analysis of post-mortem neuro-pathology studies suggests that hippocampal volume is reduced in volume in patients with mood disorders (Harrison, Brain 125:1428-1449, 2002). Hippocampal neurons are particularly susceptible to stress-related atrophy. Pathological states characterized by excessive activity within hippocampus may be improved by a therapeutic intervention that selectively reduces hippocampal excitability. Modulation of neuronal excitability within hippocampus may provide a therapeutic benefit in mood disorders.
Excess activity in hippocampus has been observed in response to emotionally-charged stimuli in bipolar patients compared to controls (reviewed by Chen et al., Bipolar Disord., 13:1-15, 2011). Chronic treatment with mood stabilizers such as lithium or valproate reduced AMPA receptor surface expression in hippocampus (Du et al., J Neurosci 28: 68-79, 2008). Tricyclic antidepressants can trigger mania in bipolar patients (Nolen and Bloemkolk, Neuropsychobiology, 42 Suppl 1:11-7, 2000); these treatments can increase AMPA receptor surface expression in hippocampus (Du et al., J Neurosci 24: 6578-6589, 2004.)
In Gray's Neuropsychological Theory of Anxiety (2003), septum and hippocampus form a ‘behavioral inhibition system’ activated during anxiety-provoking conflict situations. A corollary of this theory is that anxiolytic drugs act by suppressing this ‘behavioral inhibition system’. Indeed, intrahippocampal micro-infusion of GABAA agonists is sufficient to replicate their anxiolytic effects (Engin and Treit, Behav Pharmacol 18:365-374, 2007). Traditional anxiolytics with a variety of mechanisms-of-action, including GABAA-receptor antagonists, 5-HT1A receptor antagonists, and SSRIs, suppress brainstem-stimulated theta rhythm within hippocampus (McNaughton et al., Behav Pharmacol 18: 329-346, 2007). Direct injection of inhibitors of neuronal excitability into rodent hippocampus was shown to reduce the hippocampal theta rhythm, and to produce an anxiolytic phenotype. Intrahippocampal administration of ZD7288, an HCN channel inhibitor, slowed brainstem-stimulated theta rhythm in anesthetized rat and also increased the amount of time that rats spent in the open arms of an elevated plus maze (Yeung et al., Hippocampus 23:278-286, 2013). Intrahippocampal administration of phenytoin, a voltage-gated sodium channel inhibitor and anticonvulsant, showed similar effects on brainstem-stimulated theta rhythm frequency in anesthetized rat and was anxiolytic in conscious rat (Yeung et al., Neuropharmacology 62: 155-160, 2012).
Hippocampal overactivity has been observed in patients suffering from schizophrenia (Heckers and Konradi, Curr Top Behav Neurosci. 4:529-553, 2010). The degree of hyperactivity was be positively correlated to the severity of the symptoms (Tregellas et al., Am J Psychiatry 171: 549-556, 2014). Hypermetabolism in hippocampus (esp. CA1 region) correlates with disease progression in at-risk individuals, and with disease severity in patients diagnosed with schizophrenia (Schobel et al., Arch Gen Psych, 66:938-946, 2009). This over-activity, combined with the sensitivity of hippocampal neurons to excitotoxic damage, may lead to the observed decrease in hippocampal volume in schizophrenic patients. Neuroprotection in prodromal and early stages may prevent progressive damage (Kaur and Cadenhead, Curr Top Behav Neurosci, 2010).
In view of the clinical importance of AMPA receptors, the identification of compounds that modulate AMPA receptor function represents an attractive avenue into the development of new therapeutic agents. Such compounds are provided herein.